How lake-effect winters wear a North Side stack down
The thing that takes a North Side chimney apart is not heat and it is not age on its own. It is water that gets into the masonry and then freezes there, again and again, all winter long. Brick and mortar drink in moisture during a wet snow or a wind-driven rain off the lake, and the neighborhoods closest to the water catch more of that than most. Then the temperature falls below freezing, the trapped water swells, and it pries at the brick face and the mortar joint from the inside. A thaw lets it soak a little deeper, the next cold snap freezes it again, and over a single hard winter that cycle runs more times than anyone would guess. What you end up seeing, eventually, is brick that flakes and crumbles at the surface, joints that have hollowed out, and a crown that has split clear across. From the ground it all looks fine, which is exactly why the trouble usually goes unnoticed until a ceiling stain finally gives it away.
The way people up here actually use their fireplaces is the other half of it. North Siders burn through these winters, and a hearth that gets lit night after night in a Lincoln Park living room lays down creosote in the flue far faster than a chimney that sees a fire twice a year. Creosote is the sticky, burnable residue that condenses on the cool upper flue, and once it hardens into a glaze it is both a fire risk and a sign the flue is running cooler or dirtier than it ought to. So the same chimney that keeps a two-flat warm in January is usually the one most in need of a sweep and a careful look before the next season starts, and weighing the moisture damage against the creosote load is the heart of what we sort out on every visit.